1. Field of the Invention
The present invention relates to a carrier core material for a two-component developing agent used for developing an electrostatic latent image formed by electrophotography or electrostatic printing, a coated carrier comprising the carrier core material coated with a resin, a two-component developing agent containing the coated carrier, and an image forming method in which the two-component developing agent can be preferably employed.
2. Description of Related Art
An electrophotographic developing method is a method for developing a latent image formed on a photosensitive member by allowing toner particles of a developing agent to adhere to the latent image. The developing agents employable in this method are divided into two-component developing agents containing toner particles and carrier particles and mono-component developing agents containing toner particles only.
As a developing method using the two-component developing agent containing toner particles and carrier particles, a cascade method was used formerly, but at the present time, a magnetic brushing method using a magnetic roll is mainly used.
The carrier particle in the two-component developing agent is a carrier material, which is stirred together with the toner particles in a development box filled with the developing agent to impart desired electric charge to the toner particles and carries the charged toner particles onto the surface of a photosensitive member to form a toner image on the photosensitive member. The carrier particles held by a magnet and remaining on a developing roll return to the development box again from the developing roll and are mixed and stirred with new toner particles. Thus, the carrier particles are repeatedly used for a certain period of time.
In the two-component developing agent, the carrier particles have functions of electrostatically charging the toner particles when they are mixed and stirred with the toner particles and carrying the toner particles, differently from the mono-component developing agent, and such a two-component developing agent has good controllability in the designing of the developing agent. Therefore, the two-component developing agent is suitable for a full color developing apparatus requiring high image quality and a high-speed printing apparatus requiring reliability of image retention and durability.
In the two-component developing agent used as above, it is necessary that image properties, such as image density, fog, white spot, gradation and resolution, exhibit given values from the initial stage and these properties do not vary in the period of printing impression and stably maintained. For stably maintaining these properties, it becomes necessary that the properties of the carrier particles contained in the two-component developing agent are stable.
As the carrier particles for forming the two-component developing agent, an iron powder carrier such as an oxide-coated iron powder or a resin-coated iron powder has been conventionally used. The reason is that such an iron powder carrier has high magnetization and high electrical conductivity, so that an image having good reproducibility of a solid portion can be easily obtained. The iron powder carrier, however, is heavy in its self-weight and has too high magnetization. Therefore, extremely heavy stirring stress is applied to the toner particles in the development box, and the toner is fused to the surface of the iron powder carrier, that is, so-called toner spent is liable to occur. If the toner spent occurs, an effective surface area of the carrier is decreased, and thereby triboelectric effect (frictional charging power) due to the carrier and the toner particles tends to lower. In case of the resin-coated iron powder carrier, the resin coating is liable to peel off by the endurance stress. If the resin coating peels off, the core material (iron powder) having high electrical conductivity and low dielectric breakdown voltage is exposed, and the electrostatic latent image formed on the photosensitive member is broken by the leakage of electric charge. As a result, brush streaks are produced on the solid portion, and it becomes difficult to obtain a uniform image, that is, the resin-coated iron carrier has poor durability. For these reasons, the iron powder carriers such as the oxide-coated iron powder and the resin-coated iron powder are rarely used at present.
Instead of the oxide-coated iron powder or the resin-coated iron powder, for example, resin-coated carriers of soft ferrite, such as Cu—Zn ferrite or Ni—Zn ferrite, have been used as described in a Japanese Patent Laid-Open Publication No. 48774/1984. Because the resin-coated carrier using soft ferrite as a core material has low magnetization, a head of the developing magnetic brush can be made soft, and hence, reproducibility of vertical and horizontal lines of the resulting image becomes good.
Further, because the resin-coated carrier using soft ferrite as a core material has high dielectric breakdown voltage, leakage of electric charge rarely occurs and an image of high quality can be formed.
With increase of image quality and precision of development box, reduction of particle size of not only carrier particles but also toner particles is promoted, and toner particles having an average particle diameter of 5 to 10 μm have been mainly used as the toner particles. Further, study of use of toner particles having an average particle diameter of not more than 5 μm in the future has been started.
In order to impart desired electric charge to the toner particle of such a small diameter by friction, the carrier particle needs to have a high specific surface area, and the size of the carrier particle has been reduced. More specifically, spherical ferrite having an average particle diameter of 35 to 60 μm has been used.
Separately from the above improvements in the two-component developing agents themselves, the environment where the two-component developing agents are used has changed, and the two-component developing agents have been modified correspondingly to the change of the environment.
For example, the development system has shifted from an analog system to a digital system, or also in ordinary offices, networking has been promoted, and full color images hitherto used in the restricted sections only have been generally used recently. The full color image has a larger image area as compared with a black-and-white image generally used hitherto, and hence it is necessary to further improve properties of a solid portion and a halftone portion, such as uniformity, gradation and resolution.
In order to improve uniformity, gradation and resolution of the solid portion and the halftone portion or in order to increase a moving speed of the toner, it is advantageous to use, as the image forming method, a method of superposing AC bias on DC bias when the developing bias is applied from the magnetic brush to the, electrostatic latent image side, namely, a method of using an alternating electric field.
In the developing method using an alternating electric field, an alternating current electric field is superposed on a direct current electric field, and consequently, a strong electric field is momentarily applied to the developing agent. Because of the alternating electric field thus applied, leakage of electric charge is liable to occur, and the electrostatic latent image formed is liable to be disordered. Especially when a conventional developing agent is used, image defects such as unevenness and white spots tend to be produced in the solid portion or the halftone portion.
In order to inhibit the leakage of electric charge in a high electric field such as the alternating electric field thus applied, it is necessary to prevent dielectric breakdown of the carrier particles, and it is desirable to use, as the carrier particles, a carrier having a resin coating layer on a surface of a carrier core material. However, the amount of the resin used for the resin coated carrier is relatively small, and the thickness of the resin coating layer is thin, and besides the carrier core material is not always completely coated with the resin. Therefore, even if such a resin coating layer is formed, the leakage of electric charge is not completely prevented by the resin coating layer. That is to say, by coating the carrier particle with a resin, the carrier particle exhibits high electrical resistance in a low electric field, but in a high electric field, leakage of electric charge sometimes takes place because of an influence by the core material itself. Especially when a core material of low electrical resistance, such as iron powder or magnetite, is used as the carrier core material, such tendency is markedly observed. Further, even if the conventional Cu—Zn ferrite particles or Ni—Zn ferrite particles and the ferrite particles described in Japanese Patent Laid-Open Publication No. 69131/1996, which are considered to have relatively high dielectric breakdown voltage, are used, it is difficult to obtain uniform and faithful image reproducibility in the existing circumstances.
Japanese Patent Laid-Open Publication No. 51563/1994 and Japanese Patent Laid-Open Publication No. 35231/1994 disclose that if the magnetization of a carrier is weakened to, for example, about 30 to 150 emu/cm3, the magnetic brush becomes soft in the magnetic field at the development pole and an image faithful to the latent image can be obtained. Although the magnetic brush becomes soft and a relatively good image tends to be obtained by weakening the magnetization of the carrier as described above, image reproducibility of high level that is required with the recent increase of image quality is not satisfied.
Japanese Patent Laid-Open Publication No. 181744/1995 discloses an electrophotographic carrier obtained by treating the surface of a carrier core material with a partial hydrolysis sol such as Zr alkoxide in coating the surface of the carrier core material. This publication discloses that the thus formed coating layer is extremely hard and does not peel off during the period of service, so that a stable image can be formed. In the recent compact developing apparatus suffering heavy developing stress, however, the carrier cannot have sufficient durability in many cases, for example, because of peeling of the coating layer, the carrier core material is exposed to cause leakage of electric charge.
Japanese Patent Laid-Open Publication No. 197214/1993 discloses a carrier having a carrier core material whose surface is coated with a polyolefin resin containing carbon black, said earner core material being obtained by contacting a surface of a carrier core material with a high-activity catalyst component comprising Ti or Zr in a hydrocarbon solvent and polymerizing an olefin monomer onto the surface. In this publication, it is described that this carrier is excellent in durability, resistance to stress and resistance to environment. The carrier described in this publication, however, is a coated carrier wherein the surface of a carrier core material is coated with polyolefin, and in a high-speed apparatus suffering heavy stress, the surface coating peels off, so that sufficient durability is not obtained.
Japanese Patent Laid-Open Publication No. 194338/1996 discloses a ferrite carrier containing a specific component that is added for the purposes of maintaining high charging property of the toner and preventing adhesion of carrier and unevenness of density to improve color developing stability. In this publication, there is also disclosed a carrier wherein an oxide containing at least one element selected from Groups IA, IIA, IIIA, IVA, VA, IIIB and VB of the periodic table is added to a ferrite carrier component containing a specific component in order to control a degree of crystal growth or irregularity of a particle surface and in order to control particle density. In the case of randomly selecting such an element to be added, however, it is impossible to inhibit leakage of electric charge in a high electric field with keeping high magnetization. Further, it is very difficult to inhibit leakage of electric charge and to ensure uniformity of the solid and halftone portions in an alternating electric field or a high electric field only by such operations of controlling a degree of crystal growth or irregularity of a particle surface and controlling particle density as described in this publication. That is to say, this publication is not intended to inhibit leakage of electric charge in a high electric field, does not suggest anything about it and is entirely different from the present invention.
Japanese Patent No. 3168377 discloses that high image qualities, such as high image density, excellent highlight reproducibility and excellent fine line reproducibility, can be achieved by the use of a carrier having specific carrier resistance and specific fluidity. In this publication, it is also disclosed that this carrier is characterized in that the resistance of the carrier core material is increased by adding Bi2O3, so that when it is not added, the resistance becomes low, and if it is added too much, a uniform ferrite phase including a surface cannot be obtained. As a result, stabilization of charging of the toner cannot be achieved. Such a high-resistance carrier that is considered to be obtainable only by the addition of Bi2O3, cannot have sufficient strength against the leakage of electric charge, and besides, it is difficult to sufficiently inhibit disorder of a latent image because of low dielectric breakdown voltage. Moreover, the uniform ferrite including the surface, which is considered as excellent in this publication, cannot prevent a phenomenon of leakage of electric charge particularly in a high electric field. Hence, this carrier cannot meet the requirements of the recently increased high image qualities.